The idea of wavelength division multiplexing has been extensively developed at lower frequencies, but not at optical frequencies, and an optical wavelength division multiplexer for single-mode signals has been discussed only with respect to a device that does not provide output beams that can easily be coupled to optical fibers or to small detectors. See "Chirped-Grating Demultiplexer in Dielectric Waveguides", by A. C. Livanos et al, Applied Physics Letters, 30, 519 (1977).
The closest prior art considered and rejected the use of diffraction gratings. "Wavelength Division Multiplexing in Light Interface Technology" by R. J. Stigliani et al in Report AD-721 085, available from the National Technical Information Service, addresses the problem of designing a multimode optical multiplexer. They conclude that diffraction gratings are impractical, thus teaching away from the present invention. That report did not contemplate the use of single-mode signals.
In my copending application, Ser. No. 787,329, filed Apr. 14, 1977, now U.S. Pat. No. 4,111,524, Sept. 5, 1978, for a multimode multiplexer, a cylindrical focusing device is used to collimate and focus the multimode signals.